Currently, in the power device market, power transistors with breakdown voltage over 100V are mostly occupied by silicon base insulated gate bipolar transistor (IGBT). However, owing to the bipolar carriers characteristic of IGBT devices, the devices will suffer problems of the lifetime of the minority carriers while switching the device Consequently, if it is not possible to add lifetime killers in—during manufacturing, the system having IGBT devices would have to tolerate the power consumption and time waste while—switching IGBT devices.
By contrast, silicon base metal oxide semiconductor field transistors have a with mono-carrier species, and as a result, provide faster switch speed and less extra power consumption than those bipolar IGBTs. This is because silicon carbide has a large energy band gap of about 3.26 eV, a high critical breakdown electric field intensity and a high conductivity (4.9 W/cm-k) and is envisioned as an excellent material for the power transistor. The power transistor based on silicon carbide can achieve a breakdown voltage of 1000V without suffering any difficulty. The breakage voltage can even come up to 5 kV if the epi-layer thickness is appropriately adjusted.
Thus, it is desired to develop a silicon carbide base power transistor replacement for silicon IGBT or Schottky barrier diodes.
FIG. 6 shows an elementary structure of a Schottky diode. The structure includes a heavily-doped n-type substrate 10, an n-type epitaxial layer 20 properly doped to have the desired Schottky threshold and a silicon oxide 25 having a window for forming a Schottky contact, which is formed of a top metal layer 50. The cathode electrode 70 is formed on the rear surface of the substrate 10.
Such a structure, however, has a very poor breakdown voltage. Indeed, the equipotential surfaces tend to curve up to rise back to the surface. As a result, the curved areas of the equipotential surfaces, have very strong field variations that limit the possible reverse breakdown voltage.
A modified structure is shown in FIG. 7, in which peripheral p+ guard rings 60 are formed at the periphery of the Schottky diode area. To form the p+ guard rings 60, an additional oxide layer 30 is formed. As a result, the equipotential surfaces must pass in volume under the p regions and thus have a less pronounced curvature. This considerably improves the voltage withstand of the diode. However, the p-type guard ring cannot be made in a structure formed on a silicon carbide substrate. In fact, a diffusion annealing for a p-type dopant would require temperatures on the order of 1700° C., which raises acute technological problems.
An object of the present is thus to provide a method to overcome above problem.